1. Field of the Invention
The present invention relates to a sheet-fed printer employed for an offset press, more particularly to a sheet transferring mechanism which transfers a sheet between a plurality of printing units so as to perform multiple color press on a single surface of the sheet and a single color press on both surfaces of the sheets.
2. Description of the Related Art
Printed matters including pamphlets and catalogs are often printed using offset presses. The offset press is capable of multi-color printing and also well suited for mass printing. Sheet-fed printers, to which printing paper sheets of the same size are fed one by one to carry out offset printing, are widely employed. Such type of sheet-fed offset press is provided with a pair of impression cylinders 220, with a supply cylinder 223 being interposed therebetween via first and second feed cylinders 221,222, as shown in FIG. 24. The supply cylinder 223 has a diameter twice as great as those of the other cylinders 220,221,222.
In a mode where printing is to be applied on both surfaces of the paper Pa (double-surface printing mode or a single color press mode), the paper Pa retained on the right impression cylinder 220 is first printed on the top side by a right blanket cylinder 224a. The thus printed paper Pa retained on the supply cylinder 223 is fed through the first feed cylinder 221 to the supply cylinder 223 with the printed surface facing outward and then grasped at the front edge by grippers 230. The paper Pa is then grasped at the rear edge by a plurality of holders 225 arranged in the axial direction of the second feed cylinder 222. The paper Pa is fed further to the second feed cylinder 222 with the printed surface still facing outward to be turned over onto the left impression cylinder 220 and printed on the back side by a left blanket cylinder 224b.
In the sheet-fed printer having such reversing mechanism, when the printing paper is to be replaced with sheets of a different size, the locations of the front segment 223a and the rear segment 223b relative to the supply cylinder 223, i.e. the angle formed between the segments 223a, 223b and the axis of the supply cylinder 223 is adjusted, in such a way that the rear edge of the paper Pa may coincide with the suction heads 226 of the supply cylinder 223.
The phase of the second feed cylinder 222 relative to the supply cylinder 223 is also adjusted so that the holders 225 provided on the second feed cylinder 222 may securely grasp the rear edge of the paper Pa, and further the timing that the grippers 320 release the paper Pa is adjusted. This timing is decided by a cam ring (not shown) of the supply cylinder 223. The cam ring is fitted on the cylinder shaft of the supply cylinder 223, and a cam (not shown), which causes the grippers to release the paper Pa, is attached to the lateral surface of the cam ring. The phase of the supply cylinder 223 is adjusted by turning the cam ring in accordance with the phase adjustment of the second feed cylinder 222.
Further, as shown in FIG. 25, a pair of cams 227,228 which carry out opening and closing motions of the holders 225 are fitted side by side coaxially on the same shaft. These cams 227,228 have different cam surfaces respectively. One of them is a first control cam 227 for shifting the holding position of the paper Pa by the holders 225, and the other is a second control cam 228 for multiple color printing which is used for stably maintaining the holding position. By shifting a cam follower 229 between the position indicated by the solid line and the position indicated by the dash-dotted line, the cam follower 229 can selectively be engaged with one of the control cams 227,228. In the double-surface printing mode, the rear edge of the paper Pa must be grasped by the holders 225 provided on the second feed cylinder 222, when the paper Pa is forwarded from the supply cylinder 223 to the second feed cylinder 222; whereas, in a single-surface multiple color printing mode (or a multiple color press mode), the front edge of the paper Pa must be grasped by the holders 225. Accordingly, in order to switch from the single-surface multiple color printing mode to the double-surface printing mode, the cam follower 229 is engaged with the first control cam 227 to shift the paper holding position of the holders 225.
However, the main motor for driving the respective cylinders in the press must be stopped when the printing mode of the sheet-fed printer having such reversing mechanism, and the angles formed by the front segment 223a and the rear segment 223b with the axis of the supply cylinder 223 are manually adjusted to the scale corresponding to the size of the paper Pa. Further, the phase of the second feed cylinder 222 relative to the supply cylinder 223 is changed by loosening the bolt fastening, for example, a drive gear (or a driven gear) onto the feed cylinder to shift likewise the fixing position of the drive gear to the scale. The phase of the second feed cylinder 222 is adjusted in such a way that the holders 225 of the second feed cylinder 222 may securely hold the rear edge of the paper Pa. In the phase adjustment of the cam ring of the supply cylinder relative to the supply cylinder 223, the locking of the ring to the supply cylinder is released to turn and adjust the position of the cam ring manually to the scale in accordance with the size of the paper Pa. Further, the shifting of the paper holding position of the holders 225 is carried out by shifting manually the position of the cam follower 229.
As described above, the angle adjustment between the front segment 223a and the rear segment 223b in the supply cylinder 223, phase adjustment of the second feed cylinder 222, phase adjustment of the cam ring of the supply cylinder and shifting of the position of the cam follower 229 must totally be carried out manually. Accordingly, the mode switching from the single-surface multiple color printing to the double-surface printing or the paper size changing incurs a considerable loss of time, and further a number of defective prints are liable to occur due to mishandling in the switching operation.